1. Field of the Invention
The present invention relates to a rechargeable battery whose cathode principally comprising a nickel hydroxide (this rechargeable battery will be hereinafter referred to as a nickel series rechargeable battery) and a process for producing said nickel series rechargeable battery. More particularly, the present invention relates to a nickel series rechargeable battery having a high energy density, excelling in resistance to overcharge, and having a prolonged cycle life, wherein the cathode comprises an active material principally comprising a specific amorphous phase-bearing nickel hydroxide particulate having a prolonged lifetime and which has a high utilization efficiency of the active material (the xe2x80x9cutilization efficiency of the active materialxe2x80x9d will be hereinafter referred to as xe2x80x9cactive-material utilization efficiencyxe2x80x9d). The present invention includes a process for the production of said nickel series rechargeable battery.
2. Related Background Art
In recent years, the global warming of the earth because of the so-called greenhouse effect due to an increase in the content of CO2 gas in the air has been predicted. For instance, in thermal electric power plants, thermal energy obtained by burning a fossil fuel is being converted into electric energy, and along with burning of such fossil fuel, a large amount of CO2 gas is being exhausted in the air. Accordingly, in order to suppress this situation, there is a tendency of prohibiting to newly establish a thermal electric power plant. Under these circumstances, so-called load leveling practice has been proposed in order to effectively utilize electric powers generated by power generators in thermal electric power plants or the like, wherein a surplus power unused in the night is stored in rechargeable batteries installed at general houses and the power thus stored is used in the daytime when the demand for power is increased, whereby the power consumption is leveled.
Now, for electric vehicles which do not exhaust any air polluting substances such as CO2, NOx, hydrocarbons and the like, there is an increased demand for developing a high performance rechargeable battery with a high energy density which can be effectively used therein. Besides, there is also an increased demand for developing a miniature, lightweight, high performance rechargeable battery usable as a power source for portable instruments such as small personal computers, word processors, video cameras, and cellular phones.
In order to comply with these demands, research and development have been vigorously conducting on nickel-series rechargeable batteries in which a nickel hydroxide is used as a cathode active material, specifically nickel-metal hydride rechargeable batteries in which an anode comprising a hydrogen absorption alloy as an anode active material and a cathode a comprising a nickel hydroxide as a cathode active material are arranged through a separator having an alkaline electrolyte solution retained therein nickel-zinc rechargeable batteries in which an anode comprising a zinc material as an anode active material and a cathode comprising a nickel hydroxide as a cathode active material are arranged through a separator having an alkaline electrolyte solution retained therein. And some of these nickel series rechargeable batteries have been putted to practical use.
Incidentally, as the cathode of such nickel series rechargeable battery, a sintered type electrode has been often used. Besides, in order to more increase the battery capacity, there has proposed use of a paste-type electrode comprising a porous metallic body having a high porosity which is filled with a powdery active material of nickel hydroxide kneaded with a solution containing a binder dispersed therein as the cathode of the rechargeable battery. Now, nickel hydroxide as the cathode active material is low in terms of the conductivity and therefore, in the case where the electrode is filled substantially with nickel hydroxide only as above described, it is difficult to attain a sufficient active-material utilization efficiency. In this respect, in order for the cathode to have a sufficient active-material utilization efficiency, there has proposed a method of adding a cobalt metallic powder or a powder of a cobalt compound such as cobalt monoxide to the powdery nickel hydroxide active material upon the formation of the cathode. Here, for the case where cobalt compound is used together with nickel hydroxide in the cathode, there is generally considered such that after the cobalt compound is once dissolved in the alkaline electrolyte solution, it is oxidized upon initially subjecting the rechargeable battery to charging, followed by depositing as a highly conductive cobalt oxyhydroxide on the surface of the nickel hydroxide to form a conductive network over the surface of the nickel hydroxide. However, in the case where the cobalt metallic powder or the cobalt compound powder is added as above described, it is difficult for such cobalt metallic powder or such cobalt compound powder to be uniformly dispersed in the paste, and because of this, it is not ensured that an uniform conductive network is always formed over the surface of the nickel hydroxide.
In this respect, in order to make it possible to form an uniform conductive network over the surface of the to nickel hydroxide, there has proposed a method in which the surface of a nickel hydroxide particulate is covered by a cobalt hydroxide in advance. However, this method is not always effective for the reason that the solubility of the cobalt hydroxide in the alkaline electrolyte solution is inferior and therefore, the cobalt hydroxide is not sufficiently converted into the cobalt oxyhydroxide, where the active-material utilization efficiency is not always sufficient. In addition, the nickel hydroxide is small in terms of the oxygen overvoltage and because of this, especially when charging operation of the nickel-series rechargeable battery is performed under high temperature condition, side reaction of generating oxygen gas is liable to occur, where the charging efficiency is decreased.
In order to eliminate these problems, there have proposed a method in which a cobalt solid solution is incorporated in a nickel hydroxide crystal as the active material of the cathode and a method in which a material capable of increasing oxygen generation potential such as calcium hydroxide or yttrium oxide is added upon the formation of the cathode comprising a nickel hydroxide. However, any of these two methods is not adequate. That is, although the former method has an advantage in that the oxidation potential of the nickel hydroxide is decrease to improve the charging efficiency of the battery, it has a disadvantage in that the discharging voltage of the battery is decreased. For the latter method, it has an advantage in that the charging efficiency of the battery under high temperature condition is improved, but it has a disadvantage in that because not only the calcium hydroxide but also the yttrium hydroxide are inferior in terms of the conductivity, the active-material utilization efficiency of the cathode even under room temperature condition tends to decrease.
Separately, for a nickel series rechargeable battery whose cathode comprises an active material (that is, an active material layer) comprising a nickel hydroxide, there are disadvantages such that the active material layer of the cathode is liable to suffer from a great change in the volume (specifically, the active material layer is liable to repeatedly greatly expand and shrink) upon repeating the charging-and-discharging cycle where when the volume of the active material layer of the cathode is changed, the alkaline electrolyte solution retained in the separator situated between the anode and the cathode is likely to be absorbed by the active material layer of the cathode, resulting in shortening the lifetime of the rechargeable battery. The reason for this is considered as will be described in the following. That is, a nickel hydroxide is a crystalline material having a layer structure with a hexagonal system. As the nickel hydroxide used as the cathode active material of the nickel series rechargeable battery, there is usually used a xcex2-type nickel hydroxide. In this case, the inter-layer distance of the xcex2-type nickel hydroxide as the cathode active material is about 0.46 nm. And, the inter-layer distance of a xcex2-type nickel oxyhydroxide as a product provided when the xcex2-type nickel hydroxide as the cathode active material is subjected to charging is about 0.48 nm. However, upon subjecting the xcex2-type nickel hydroxide as the cathode active material to charging, a xcex3-type nickel oxyhydroxide having a structure in which alkali metal ion or water molecule originated from the alkaline electrolyte solution is entrapped is additionally produced as a by-product, where the inter-layer distance of the xcex3-type nickel oxyhydroxide is about 0.69 nm. Incidentally, it is known that the xcex3-type nickel oxyhydroxide as the by-product is more liable to generate when the rechargeable battery is over-charged. Now, under such condition where the T-type nickel oxyhydroxide as the by-product is generated, upon the repetition of the charging-and-discharging cycle, because the volume of the active material layer of the cathode is greatly changed (expanded and shrunk), there is a tendency in that the number of micropores present in the particulate of the nickel hydroxide is gradually increased as the charging-and-discharging cycle is progressed and as a result, the particulate of the nickel hydroxide is collapsed. In order to prevent this problem from being occurred, there is known a method in which cadmium or zinc in a solid solution state is incorporated in the crystalline structure of the nickel hydroxide (the xcex2-type nickel hydroxide).
Now, a nickel hydroxide (a xcex2-type nickel hydroxide) can be prepared by a reactive crystallization method wherein an aqueous solution of an alkali such as sodium hydroxide is dropwise added to an aqueous solution containing a nickel salt dissolved therein while stirring the nickel salt aqueous solution to precipitate a nickel hydroxide (a xcex2-type nickel hydroxide). In this case, by introducing a prescribed amount of a prescribed cadmium salt or a prescribed zinc salt, it is possible to obtain a nickel hydroxide particulate incorporated with cadmium or zinc. When this nickel hydroxide particulate is used as the active material of the foregoing cathode, it will be possible to restrain the generation of a xcex3-type nickel oxyhydroxide in ordinary charging or discharging region. However, it is difficult to ensure that the generation of the xcex3-type nickel oxyhydroxide in over-charging region is sufficiently restrained. Separately, in order to improve the effect of restraining the generation of the xcex3-type nickel oxyhydroxide, when a nickel hydroxide particulate obtained by increasing the amount of the cadmium salt or the zinc salt introduced in the above method is used as the active material of the cathode, a problem entails such that the relative amount of the nickel hydroxide (the xcex2-type nickel hydroxide) in the active material of the cathode is decreased, where it is difficult to make the cathode have a high capacity.
In this respect, as other method of restraining the generation of the xcex3-type nickel oxyhydroxide, there is known a method in that the crystallinity of the nickel hydroxide is decreased. For instance, Japanese Unexamined Patent Publication No. 172563/1998 discloses a method in which in the course of preparing a nickel hydroxide by such reactive crystallization method as above described, by properly controlling the pH value, temperature, agitation speed, and the like of the reaction solution, the crystallinity of a nickel hydroxide particulate obtained is decreased. Besides, Japanese Unexamined Patent Publication No. 50307/1998 discloses a method in which by adding a mechanical energy comprising a compression force and a frictional force to the surface of a nickel hydroxide particulate, the crystallinity of the nickel hydroxide particulate is decreased. Here, the nickel hydroxide particulate obtained by the former method has a half-value width of a diffraction peak of a (101) crystal face in X-ray diffraction using Kxcex1-rays of Cu as a radiation source which is about 0.9xc2x0, and similarly, the nickel hydroxide particulate obtained by the latter method has a half-value width, which is about 1.0xc2x0. Thus, even by these methods, it is difficult to decrease the crystallinity of the nickel hydroxide particulate to a level capable of sufficiently restrain the generation of the xcex3-type nickel oxyhydroxide. Thus, it is understood that these methods are difficult to afford an effect apparently superior to that by the method by way of introducing cadmium or zinc in a solid solution state.
Incidentally, there has been attempted to improve the capacity of the cathode active material itself by positively utilizing the xcex3-type nickel oxyhydroxide.
Here, the nickel valence number of the foregoing xcex2-type nickel hydroxide is 2.1, and that of the foregoing xcex2-type nickel oxyhydroxide is 3.1. And the charge-and-discharge reaction between the foregoing xcex2-type nickel hydroxide and the xcex2-type nickel oxyhydroxide becomes to be a 1.0 electron reaction which is subtracting said 2.1 from said 3.1. On the other hand, the nickel valence number of the xcex3-type nickel oxyhydroxide is approximately 3.5.
In the light of this situation, when the charge-and-discharge reaction between the xcex2-type nickel hydroxide and the xcex3-type nickel oxyhydroxide is considered, the charge-and-discharge reaction becomes to be a 1.4 electron reaction which is subtracting said 2.1 from said 3.5.
From this, it is thought that if the xcex3-type nickel oxyhydroxide can be efficiently produced, it will be possible to improve the active-material utilization efficiency until 140%.
Now, there are proposals to use a xcex3-type nickel oxyhydroxide in a nickel series rechargeable battery. For instance, Japanese Unexamined Patent Publication No. 172561/1998 discloses a rechargeable battery whose cathode has an active material layer composed of an xcex2-type nickel hydroxide which is approximate a xcex3-type nickel oxyhydroxide in terms of the inter-layer distance. However, this rechargeable battery has a drawback in that the xcex1-type nickel hydroxide as the cathode active material is readily oxidized into a xcex3-type nickel oxyhydroxide when the rechargeable battery is subjected charging, where the capacity of the cathode active material layer is increased but the density of the cathode active material itself is low, and therefore, the cathode active material layer is insufficient in terms of the density. Besides, Japanese Unexamined Patent Publication No. 289714/1998 discloses a rechargeable battery whose cathode has an active material layer composed of a nickel hydroxide particulate obtained by admixing manganese (Mn), aluminum (Al) or chromium (Cr) in a solid solution state in a nickel hydroxide crystalline. Although this rechargeable battery has an advantage in that an improvement in the active-material utilization efficiency of the cathode is attained at an initial stage of the charging-and-discharging cycle, it has disadvantages in that the volume change (the expansion and shrinkage) of the active material layer of the cathode is difficult to be essentially improved, and because of this, it is difficult to sufficiently prolong the charging-and-discharging cycle life.
The present invention has been accomplished in view of the foregoing situation in the prior art for the nickel series rechargeable batteries whose cathode comprising nickel hydroxide.
An object of the present invention is to provide a nickel series rechargeable battery whose cathode having an active material layer formed of a specific nickel hydroxide particulate and which has an improved active-material utilization efficiency and a high energy density, excels in resistance to overcharge (hereinafter ref erred to as overcharge resistance), and has a prolonged charging-and-discharging cycle life.
Another object of the present invention is to provide a process for producing said rechargeable battery.
The rechargeable battery provided according to the present invention includes the following three embodiments.
A first embodiment of the rechargeable battery according to the present invention is a rechargeable battery comprising at least a cathode, an anode, a separator, and an electrolyte comprising an alkali electrolyte solution, said cathode comprising an active material layer which participates in battery reaction and a collector, characterized in that said active material layer of said cathode comprises a material containing an amorphous phase-bearing nickel hydroxide particulate which in X-ray diffraction using Kxcex1-rays of Cu as a radiation source, has a diffraction peak of a (001) face appeared near a diffraction angle 2xcex8=19xc2x0 having a half-value width of more than 1.2 and has a diffraction peak of a (101) face appeared near a diffraction angle 2xcex8=38xc2x0 having a half-value width of more than 1.5xc2x0. The amorphous phase-bearing nickel hydroxide particulate is also featured that a crystallite size in a direction perpendicular to the (001) face and a crystallite size in a direction perpendicular to the (101) face which are calculated from the result of the X-ray diffraction are respectively less than 8 nm. The amorphous phase-bearing nickel hydroxide particulate is further featured that it comprises particles in an undefined form having an average particle size in a range of 0.2 to 2 xcexcm. The amorphous phase-bearing nickel hydroxide particulate may contain Zn or/and Cd respectively as a minor component. The amount of Zn or/and Cd to be contained is 0.2 wt. % or less on the basis of the amount of the hydroxide.
A second embodiment of the rechargeable battery according to the present invention is that the active material layer of the cathode comprises the above-described amorphous phase-bearing nickel hydroxide particulate, an electrically conductive material comprising a metallic cobalt or/and a cobalt compound, and an additive comprising at least one kind of a metal compound selected from the group consisting of alkaline earth metal compounds, rare earth metal compounds, transition metal compounds of transition metal elements belonging to groups 4B, 5B, 6B, and 7B of the periodic table, and metal compounds of metal elements belonging to group 3A of the periodic table. In this case, it is possible for the electrically conductive material or/and the additive to be contained such that they cover part of or the entirety of the surface of the amorphous phase-bearing nickel hydroxide or they are combined with the amorphous phase-bearing nickel hydroxide.
The addition amount of the electrically conductive material is preferred to be in a range of 5 to 20 wt. % versus the total amount of the constituents of the active material layer. The cobalt compound as the electrically conductive material can include cobalt monoxide, cobalt hydroxide, and cobalt oxides having an alkali metal contained therein. The alkali metal can include K, Na, and Li. The addition amount of the additive is preferred to be in a range of 1 to 5 wt. % versus the total amount of the constituents of the active material layer. The alkaline earth metal compound as the additive can include oxides and hydroxides of calcium (Ca), oxides and hydroxides of magnesium (Mg), oxides and hydroxides of strontium (Sr), and oxides and hydroxides of barium (Ba). The rare earth metal compound as the additive can include oxides and hydroxides of yttrium (Y), oxides and hydroxides of holmium (Ho), oxides and hydroxides of erbium (Er), oxides and hydroxides of thulium (Tm), oxides and hydroxides of ytterbium (Yb), and oxides and hydroxides of lutetium (Lu). The transition metal compound as the additive can include oxides and hydroxides of titanium (Ti), oxides and hydroxides of vanadium (V), oxides and hydroxides of chromium (Cr), and oxides and hydroxides of manganese (Mg). The group 3A compound as the additive can include oxides and hydroxides of aluminum (Al), oxides and hydroxides of gallium (Ga), and oxides and hydroxides of indium (In). Of these metal compounds as the additive, oxides and hydroxides of yttrium (Y), oxides and hydroxides of ytterbium (Yb), oxides and hydroxides of calcium (Ca), and oxides and hydroxides of aluminum (Al) are particularly preferable.
A third embodiment of the rechargeable battery according to the present invention is that the active material layer of the cathode principally comprises the foregoing amorphous phase-bearing nickel hydroxide particulate described in the first embodiment and a crystalline nickel hydroxide particulate which in X-ray diffraction using Kxcex1-rays of Cu as a radiation source, has a diffraction peak of a (001) face appeared near a diffraction angle 2xcex8=19xc2x0 having a half-value width of less than 0.8xc2x0 and has a diffraction peak of a (101) face appeared near a diffraction angle 2xcex8=38xc2x0 having a half-value width of less than 1.1xc2x0.
The crystalline nickel hydroxide particulate is preferred to comprises particles in a substantially spherical form having an average particle size in a range of 5 to 30 xcexcm. The average particle size of the crystalline nickel hydroxide particulate is preferred to be 5 times or more that of the amorphous phase-bearing nickel hydroxide particulate.
The crystalline nickel hydroxide particulate is preferred to contain at least one kind of an element selected from the group consisting of Zn, Mg and Ba in a solid solution state.
As above described, the present invention provides a process for producing a nickel series rechargeable battery whose cathode having an active material layer formed of a specific nickel hydroxide particulate and which has an improved active-material utilization efficiency and a high energy density, excels in overcharge resistance, and has a prolonged charging-and-discharging cycle life.
The rechargeable battery-producing process includes the following three embodiments.
A first embodiment is a process for producing a rechargeable battery comprising at least a cathode, an anode, a separator, and an electrolyte comprising an alkali electrolyte solution, said cathode comprising an active material layer which participates in battery reaction and a collector, characterized in that said active material layer of said cathode is formed by using (a) an amorphous phase-bearing nickel hydroxide particulate which in X-ray diffraction using Kxcex1-rays of Cu as a radiation source, has a diffraction peak of a (001) face appeared near a diffraction angle 2xcex8=19xc2x0 having a half-value width of more than 1.2xc2x0 and has a diffraction peak of a (101) face appeared near a diffraction angle 2xcex8=38xc2x0 having a half-value width of more than 1.5xc2x0, (b) an electrically conductive material comprising a metallic cobalt or/and a cobalt compound, and (c) an additive comprising at least one kind of a metal compound selected from the group consisting of alkaline earth metal compounds, rare earth metal compounds, transition metal compounds of transition metal elements belonging to groups 4B, 5B, 6B, and 7B of the periodic table, and metal compounds of metal elements belonging to group 3A of the periodic table. The amorphous phase-bearing nickel hydroxide particulate (a) can be prepared by subjecting a prescribed nickel hydroxide powder to a mechanical grinding treatment. The mechanical grinding treatment is preferred to be conducted by using a grinding apparatus such as planetary ball mill, tumbling ball mill, or vibration ball mill. Besides, the amorphous phase-bearing nickel hydroxide particulate (a) can be prepared by a method wherein a solution containing a prescribed nickel compound dissolved therein is mixed with a chelating agent or a surface-active agent with a concentration which is greater than a critical micelle concentration and the mixture is reacted with an alkali. As the nickel compound, nickel nitrate, nickel chloride, nickel carboxylate, and nickel alkoxide can be selectively used. As the chelating agent, citric acid, tartaric acid, maleic acid, or acetylacetone can be used.
A second embodiment of the rechargeable battery-producing process is characterized in that in the first embodiment, the amorphous phase-bearing nickel hydroxide particulate (a) comprises particles having a surface which is partially or entirely covered by the above-described electrically conductive material (b) or/and the above-described additive (c) or the amorphous phase-bearing nickel hydroxide particulate (a) is combined with the electrically conductive material (b) or/and the additive (c) into a composite. The amorphous phase-bearing nickel hydroxide particulate (a) partially or entirely covered by the electrically conductive material (b) or/and the additive (c) or the composite comprising the amorphous phase-bearing nickel hydroxide particulate (a) and the electrically conductive material (b) or/and the additive (c) may be prepared by mechanically mixing (a) a prescribed nickel hydroxide particulate prior to amorphization, (b) a prescribed electrically conductive material or/and (c) a prescribed additive using a grinding apparatus such as planetary ball mill, tumbling ball mill, or vibration ball mill.
Besides, a material comprising the amorphous-phase bearing nickel hydroxide particulate (a) covered by the electrically conductive material (b) may be prepared by a manner in that a prescribed amorphous phase-bearing nickel hydroxide particulate (powder) is dispersed in a treating solution containing at least a prescribed cobalt salt dissolved therein, followed by being reacted with at least one kind of a compound selected from the group consisting of potassium hydroxide, sodium hydroxide, and lithium hydroxide. When the resultant obtained here is immersed in a solution containing at least one kind of a compound selected from the group consisting of potassium hydroxide, sodium hydroxide, and lithium hydroxide dissolved therein, followed by subjecting to a heat treatment in the presence of oxygen, there can be formed an amorphous phase-bearing nickel hydroxide particulate (a) covered by a highly electrically conductive coat layer. The cobalt salt contained in the above treating solution can include cobalt sulfate, cobalt nitrate, and cobalt chloride. It is possible for the treating solution to additionally contain at least one kind of a metal compound selected from the group consisting of alkaline earth metal compounds, rare earth metal compounds, transition metal compounds of transition metal elements belonging to groups 4B, 5B, 6B, and 7B of the periodic table, and metal compounds of metal elements belonging to group 3A of the periodic table.
Separately, when a prescribed amorphous phase-bearing nickel hydroxide particulate (a) is mixed with a cobalt salt capable of being decomposed at a temperature which is lower than the decomposition temperature of the nickel hydroxide and the mixture is heated until a temperature where the cobalt salt is decomposed, there can be formed an amorphous phase-bearing nickel hydroxide particulate covered by an electrically conductive material. As the cobalt salt, cobalt nitrate is preferable. When the resultant obtained in this case is immersed in a solution containing at least one kind of a compound selected from the group consisting of potassium hydroxide, sodium hydroxide, and lithium hydroxide dissolved therein, followed by subjecting to a heat treatment in the presence of oxygen, there can be formed an amorphous phase-bearing nickel hydroxide particulate (a) covered by a highly electrically conductive coat layer.
A third embodiment of the rechargeable battery-producing process is characterized in that in the first embodiment, in addition to the amorphous phase-bearing nickel hydroxide particulate (a), the electrically conductive material (b) or/and the additive (c), there is used a crystalline nickel hydroxide particulate (d) which in X-ray diffraction using Kxcex1-rays of Cu as a radiation source, has a diffraction peak of a (001) face appeared near a diffraction angle 2xcex8=19xc2x0 having a half-value width of less than 0.8xc2x0 and has a diffraction peak of a (101) face appeared near a diffraction angle 2xcex8=38xc2x0 having a half-value width of less than 1.1xc2x0. The crystalline nickel hydroxide particulate (d) is preferred to comprises particles in a substantially spherical form and have an average particle size which is 5 times or more that of the amorphous phase-bearing nickel hydroxide particulate (a).
The crystalline nickel hydroxide particulate (d) is preferred to comprises particles in a substantially spherical form having a surface which is partially or entirely covered by the electrically conductive material (b) or/and the additive. It is preferred that the amount of the crystalline nickel hydroxide particulate (d) to be added is controlled to fall in a range of from 10 to 70 wt. % versus the total amount of the constituents of the active material layer of the cathode.
The crystalline nickel hydroxide particulate (d) is preferred to contain at least one kind of an element selected from the group consisting of Zn, Mg and Ba in a solid solution state.
The cathode in the present invention may be formed, for instance, in the following manner. A prescribed amorphous phase-bearing nickel hydroxide particulate (a), or said amorphous phase-bearing nickel hydroxide particulate (a) and a prescribed crystalline nickel hydroxide particulate (d), a prescribed electrically conductive material (b), and a prescribed additive (c) are mixed to obtain a mixture, the mixture is mixed with a solution containing a binder dissolved therein to obtain a paste, and the paste is applied to a porous metal body formed of a nickel material or a nickel-plated metallic material or a nonwoven member formed of a metallic fiber as a collector such that the porous metal body or the nonwoven member as the collector is impregnated with the paste. Alternatively, the cathode may be formed by arranging the paste on the surface of a punching metal member, an expanded metal member or a metal foil respectively comprising a nickel material or a nickel-plated metallic material as a collector to form a layer as the active layer of the cathode. In the latter case, if necessary, the paste may be added with an electrically conductive auxiliary in a flake form, a spherical form, a filament form, a needle form, or a spike form, comprising at least one kind of a powdery material selected from the group consisting of a nickel powder, a copper powder, and a carbon powder. The binder of the paste can include methyl cellulose, carboxymethylcellulose, and polyvinyl alcohol.
As the alkali electrolyte solution used in the rechargeable battery of the present invention, it is preferred to use an aqueous solution containing potassium hydroxide dissolved therein at a concentration in a range of from 8 to 12 mol/l. The potassium hydroxide aqueous solution as the alkali electrolyte solution may contain lithium hydroxide or/and sodium hydroxide.
In the rechargeable battery-producing process of the present invention, after a rechargeable battery is produced using the foregoing cathode, an anode, a separator, and aforesaid alkali electrolyte solution, the rechargeable battery is over-charged so that the rechargeable battery is charged with an electricity quantity corresponding to 200% or more of the capacity of the cathode, and thereafter, for the rechargeable battery thus over-charged, discharging is preformed until the voltage of the rechargeable battery reaches a prescribed battery voltage. The operation is conducted at least one or more times. By performing this operation, the rechargeable battery is deeply charged and discharged while preventing occurrence of xcex3-type nickel oxyhydroxide, where the nickel hydroxide as the active material of the cathode is effectively activated to exhibit an improved active-material utilization efficiency in the charging-and-discharging cycle thereafter.
According to the present invention, by using a cathode formed using a specific amorphous phase-bearing nickel hydroxide particulate (or a specific amorphous nickel hydroxide particulate), there can be attained a rechargeable battery (specifically, a nickel-metal hydride rechargeable battery) which is provided with an improved cathode which is high in terms of the packing density of the active material and has a high active-material utilization efficiency, and which excels in overcharge resistance and a prolonged charging-and-discharging cycle life. The present invention is applicable also in other nickel series rechargeable batteries such as a nickel-zinc rechargeable battery, a nickel-cadmium rechargeable battery, and the like.